Tracking issue for more slice patterns

[carols10cents]

Rust tracking issue: rust-lang/rust#23121

Place that will need to be updated: https://github.com/rust-lang-nursery/edition-guide/blame/68c458b9b68edced39aebea60e672a22d4d50edf/src/rust-2018/slice-patterns.md#L86-L91

In the pipeline

When it comes to slice patterns, more advanced forms are planned but
have not been stabilized yet. To learn more, follow the tracking issue.

[Centril]

New tracking issue is rust-lang/rust#62254.

[shepmaster]

Proposed stabilization PR at rust-lang/rust#67712

[ehuss]

Closing per #232, this section has been removed. Slice patterns were added to the reference in rust-lang/reference#876, which unfortunately isn't very friendly for beginners, but that's what we have for now.